A machining center (MC) is a machine which automatically selects various tools according to machining steps and automatically mounts these tools on a main spindle so as to perform various kinds of machining. In the MC, a tool change is made by an automatic tool holder changing (ATC) device, and the ATC device automatically takes out a tool holder, to which a tool is attached, from a tool magazine and automatically mounts the tool holder on the main spindle.
As shown in FIG. 14(a), a tool holder 2 holding a tool 1 has a conical engaging portion 2A, and mounting is performed by engaging the engaging portion 2A in a conical engaged portion 3A formed in a main spindle 3; however, as shown in FIG. 14(b), if chips 4, etc. are caught at these engagement portions, the mounting is performed in a state where the axis is displaced. If machining is then performed in this state, vibration occurs in the tool 1, generating the defect that the machining accuracy of the work decreases greatly.
Conventionally, such a chuck error of a tool holder is detected, for example, by casting laser light upon the tip of the tool on the tool holder mounted on the main spindle to detect whether the tip of the tool is present at a prescribed position. That is, the tip of the tool should always be present at the prescribed position if there is no chuck error in the tool holder, and a case where the tip of the tool is not at the prescribed position is judged to be a chuck error.
However, the conventional method by which whether the tip of the tool is at a prescribed position is detected by use of laser light has the disadvantage that detection errors are liable to occur when a large amount of coolant is used as in an MC because laser light is apt to be shut off by the coolant.
The present invention has been made in consideration of such a situation, and has as its object the provision of a machine tool which can easily and positively detect chuck errors.